Weldable silver-graphite contact and method of making it



S 9, 1- E. F. KIEFER ETAL 2,255,120 -Y WELDABLE SILVER-GRAPHITE CONTACT AND METHOD OF- MAKING IT Filed Oct. 15, 1959 Patented Sept. 9, 1941 WELDABLE SILVER-GRAPHITE CONTACT AND mz'rnon or MAKING rr Edwin F. Kiei'er and Eric I. Shobert, 11, St. Marys, Pa., assignors to Stackpolc Carbon Company, St. Marys, Pa., a corporation of Pennsylvania Applicationoctober 13, 1939, Serial No. 299.330

. 10 Claims. 7 This invention relates to silver-graphite electric contact elements forv electric switches, by

which are meantany electric circuit making and breaking devices.

Silver-graphite contact elements are usually madeby molding under high pressure a mixture of powdered silver and graphite. The silver 1 forms the principal conductor for the'electric current, and the graphite is added because it is element which it engages.

a refractory material which prevents the contact element from welding to the other switch contact However, in producing a non-welding contact of this character it necessarily follows that it is very diflicult to weld the contact to the "arm or other member which cent of silver powder and about 5 per cent of graphite powder which have been "thoroughly supports it andiforms part of the electric circuit.

It is an object of this invention to provide a silver-graphite contact element which retains its non-welding characteristics in service but which can be readily welded to a supporting member. Another object is to provide a simple and inexpensive method for making such a contact element.

An embodiment of the inventicii is illustrated in the accompanying drawing in which 1 is a verticalsection through a perspective-viewof a silver-graphite contact element; Fig-.1 2 .is' a viewtsimilar to Fig. 1 of our contact-element after it has been treated in'accordancwith the invention disclosed herein; and Fig. 31s a vertical cross section through our element welded .to a supporting arm'of an electric switch.

The non-welding silver-graphite contact ele-- ment I shown in Fig. 1 maybe-made in'any suit.- able manner, but preferably from about 95 per mixed together and then molded under high pressure into a block as shown. Such a contact will notiuse or-weld to the other contact element with which it is moved into and out of engagementin service, but it is very diflicult to weld the element to the member which supportsit. :L

In accordance with this invention this dimcu'lty is overcome by oxidizing the graphite from the surface of thecontact element that is to be welded to the supporting arm so as to leave a readily weldable silver surface substantially free of graphite. Preferably, this is done by heating that surface at a temperature of about 1200" F.

for a period of about two minutes in air, although the temperature and the time of heating required to produce the best'weldin'g surface will depend the type of atmosphere which is used to oxidize the graphite. By this method must of the graphite in the heated surface of the contact element is oxidized away, leaving a clean surface of silver 2 which can easily be welded 5 to a supporting arm 3, as shown in Fig. 3.

It is desirable to protect. the circuit making and breaking or contact face of the element from the oxidizing atmosphere applied to the other surface so that the contact face will retain its 10 non-welding properties:" As shown in Fig. 2, this is preferably done by covering the contact face of the element with a carbon plate 4. In spite of thisprotection a very thin layer 5 of the plateengaging surface of the element may be oxidized,

but this thin layer can be readily ground oil so that the contact face will be composed of bothgraphite and silver. The oxidized ,or welding surface of-the contact element may be suitably marked so that it can be identified when it is desired to weld the element to a supporting.

member.

. This process may be carried out in such a manner that not only is the graphite oxidized from the surface to be welded, but the contact material is baked or sintered at the same time.

Another method of oxidizing the graphite from one surface of the contact element is to place a small piece of silver nitrate on thatsurface and then heat the nitrate and contact element together; When heated, the silver nitrate gives of! nitrous oxides which oxidize the graphite from the adjoining surface of 'the contact element.

' In addition, the silver nitrate decomposes and deposits additional silver on the element. To

make the welding surface stronger it may be desirable to increase the temperature at which the element is heated to a point wherethe silver decomposed from the silver nitrate sinters itself to the silver forming the welding surface of the 4 contact element.

According to the provisions of thepatent statutes, we have explained the principle and construction of our invention and haveillustrated and described what we now consider to represent 5 its best embodiments. However, we desire to.

have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described. 1

We claim:. 1

1. A silver-graphite electric contact element having one surfacefrom which the graphite has been oxidized to leave a readily weldable silver face.

2. A silver-graphite electric contact element consisting of a compacted body of intermixed powdered silver and graphite and having one surface from which. the graphite has been removed to leave a readily weldable-silver face.

3. The method of making a weldable silvergraphite electric contact element, comprising oxidizing the graphite from a surface of. the ele- .ment to thereby leave a readily weldable silver face substantially free of graphite.

4. The method of making arweldable silvergraphite electric contact element, comprising heating a surface of the element at approximately 1200 F. for approximately two minutes in air to oxidize the graphite therefrom and thereby leave a readily weldable silver face substantially free o'f g-raphite.

5 .-I'he method of making a weldable silvergraphite electric contact element, comprising heating .a surface of the element in an oxidizing atmosphere to oxidize the graphite therefrom and thereby leave a readily weldable silver facesubstantially free of graphite, and. protecting the contact face of the element from said oxidation.

'6. The method of making a weldable silvergraphiteelectric contact element,- comprising placing the contact face of the element against a carbon plate, and heating an exposed surface of 8. The method of making a weldable silvergraphite electric contact element, comprising placing silver nitrate on a surface of the element, and heating said nitrate and element to oxidize the graphite from said surface and to add silver thereto, the temperature of said heating being suflicient to sinter the added silver to the silver of the contact element.

9. The method of making electric contacts which comprises mixing graphite and contact metal powders, pressing the mixture into a body of suitable shape, and thensintering said body in an oxidizing atmosphere while shielding the contact face of said body from said atmosphere,

whereby to produce a sintered 'contact body having graphite at and adjacent the contact 1 face thereof and substantially devoid of graphite at the element in an oxidizing atmosphere to oxidize the graphite therefrom and thereby leave a readily weldable face of silver.

'7. The method of making a weldable silvergraphite electric contact element, comprising placing silver nitrate on a surface of the element,

and heating said nitrate and element to oxidize the graphite from said surface and to add-silverthereto.

the back face thereof, and then securing said 'contact 'body'to a support by fusing a binder metal between said back face and a support and allowing the binder to solidify.

10. The methodv of making electriccontact discs off-graphite interspersed and, bonded with silver base contact metal which comprises mixing graphite and silver base metal powders, pressing the same into discs, placing said discs with'one face thereof against a-surface of a carbonaceous body and then sintering said discs with the opposite faces of said discs exposed to anfoxidizing atmosphere, whereby to produce sintered contact discs having graphite at and adjacent to the I contact face thereof and-substantiallydevoid of graphiteahtheback face thereof.

ERLE 1. snonna'r, n. 

